HOUSTON, July 23 (Reuters) - Tropical Storm Bonnie could help dissipate a giant oil slick in the Gulf of Mexico, but could further damage fragile marshlands if heavy wind and waves drive more oil ashore, experts said on Friday.

The fast-moving Bonnie is over south Florida on a track that is expected to take it near the site of BP Plc’s (BP.L) (BP.N) massive oil leak, and make landfall on the Louisiana coast late on Saturday or early Sunday.

The collision of a tropical storm with a giant oil spill is an unprecedented event, and experts predicted both positive and negative impacts.

“It might actually be good for cleansing the system but in other circumstances it might cause even more problems if it blows a lot of the oil directly onshore,” said Chuck Kennicutt, an oceanography professor at Texas A&M University in College Station, Texas.

A storm’s intense wave action can accelerate the natural process of breaking down the oil, causing it to evaporate or coagulate into emulsified tarballs, Kennicutt said.

But a storm’s natural tidal surge could pound the oil into fragile marshes and bayous, where they will be difficult to clean up, he said.

“We’re pretty much in unknown territory here,” he said.

Kennicutt’s assessment was echoed by the top U.S. oil spill official on Friday.

“I think there is a good part and a bad part to that,” retired Coast Guard Admiral Thad Allen said, referring to the storm.

“Sometimes the increased action on the surface can actually help with the emulsification of the oil and the distribution and biodegradation,” Allen said. “On the other hand you have the chance that a storm surge can drive that up into the beach and marshes, where it would not have been driven otherwise.”

The U.S. Coast Guard estimated about half the oil from the spill has evaporated or dissipated already.

A BIG BOOM

High winds and waves could displace hundreds of miles of floating plastic boom placed around the most sensitive beaches and wetlands, including key bird habitats.

“That’s something that you can anticipate — the boom just getting blown everywhere,” said Mike Parr, vice president of the American Bird Conservancy.

High winds could blow an oily water mix over sandy berms and into marshes that are key habitats for birds like the reddish egret and the black skimmer, Parr said.

In a potentially positive sign, Allen said that the amount of oil observed at the spill site has decreased in recent days.

“There’s not a lot of oil out there,” Allen said.

In some cases, a fleet of about 800 skimming vessels has had a hard time finding oil to clean, Allen said this week.

But according to U.S. government estimates cited by Louisiana Gov. Bobby Jindal this week, there are still about 1.6 million barrels of oil sloshing around in the Gulf.

The U.S. Coast Guard estimated about 5.4 million barrels of oil has spilled into the Gulf since the April 20 rig explosion and, of that, about 2.6 million barrels have evaporated or biodegraded. That was based on U.S. scientists’ estimates that the Macondo well had spewed up to 60,000 barrels (2.5 million gallons/9.5 million liters) a day before being sealed on July 15.

The worst offshore oil spill in U.S. history also has soiled about 475 miles (764 km) of Louisiana shoreline, Jindal said. (Reporting by Chris Baltimore, Editing by Erwin Seba and Jackie Frank)